This application is the national phase under 35 U.S.C. xc2xa7371 of prior PCT International Application No., PCT/GB97/00400, which has an International filing date of Feb. 10, 1997, which designated the United States of America, the entire contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a cementitious tile having good acoustic properties, to a method of making such a tile and to a die assembly for use in the method.
2. Description of the Relevant Art
Board made from gypsum plaster is generically termed plasterboard. Conventional paper faced plasterboard is used as a cladding for building interiors, either to give, or to provide a base for, the desired decorative finish.
Plasterboard has been successfully used in other applications, such as ceiling tiles, but has not generally been very successful in applications where good acoustic absorption properties are required. GB-A-2 203 772 discloses a plasterboard having relatively good acoustic absorption properties. The board is perforated by holes or slits which are covered on one face of the board by cloth bonded to the board. WO-A-87/00116 discloses a plasterboard for use as an acoustic tile perforated with regular slots. It has been desired to improve the acoustic absorption properties of plasterboard tiles; it has also been desired to achieve this in a tile of esthetically pleasing appearance.
According to the present invention, there is provided a sound absorbent tile comprising cementitious material having through perforations which extend through cementitious material, characterised in that at least some of the perforations are fissure-like perforations. Preferably some of the perforations are circular in section.
By xe2x80x9cfissure-likexe2x80x9d is meant generally elongate perforations having irregular edges, preferably with an aspect ratio (the ratio of the length of the fissure to its maximum width) of at most 4:1. An aspect ratio of at least 2:1 is preferred.
Preferably, the tile is lined, for example with a paper liner and the lined surface has an array of indentations which extend through the liner and terminate in the cementitious material. The liner of the plasterboard is ruptured, giving rise to a product of distinctive appearance. The liner is forced into the indentations during their formation giving rise to a level of contrast in between the two extremes produced by the machining operations described previously.
Preferably, the openings of the through perforations on the side of the board opposite the lined surface (if any) are covered. In an especially preferred embodiment, these openings are covered with a sound absorbent material, preferably in sheet form such as acoustic paper or felt.
Also according to the invention there is provided a method of manufacturing a sound absorbent tile of cementitious board comprising:
contacting a planar surface of a cementitious board with the profiled surface of a punch plate having punches thereon, the shapes of at least some of the punches being such as to form fissure-like perforations;
perforating the board with fissure-like perforations by applying pressure between the board and the die such that the punches pass through the board; and
thereafter separating the punch plate from the board.
Preferably, the profiled surface of the punch plate includes indentors of similar shape to the punches, shorter than the punches, the method comprising embedding the punches and the indentors in the board so that the punches through the board and the indentors penetrate but do not pass through the board.
If the board is lined, it is preferred that the punch plate impinges on the lined surface.
In a preferred method, the planar surface of the board is painted after the board has been punched and indented. In this way, any liner forced into the indentations can be left unpainted, particularly if the paint is applied with a roller for example. Painting provides a way of varying the degree of contrast between the indentations and the rest of the board.
In an especially preferred method, the surface of the board is spiked using a roller having radially extending spikes. Spiking can be used to produce fine pinholes in the surface of the board which gives a particularly pleasing appearance in combination with the fissure-like perforations and indentations.
Also according to the invention there is provided a die assembly for use in perforating cementitious board, comprising a punch plate and punches arranged in an array on the surface of the punch plate, the punches each having a substantially elongate fissure-like profile, to form fissure-like perforations in a board. Preferably, the die assembly further comprises indentors of similar shape to the punches for penetrating a cementitious board, the inventors being arranged on the surface of the punch plate and extending a smaller distance from the said surface than do the punches. Particularly preferably, the punch plate also carries circular punches for producing circular perforations in the board in addition to the fissure-like perforations.
It is preferred that the punches extend beyond the indentors by an amount at least as great as the thickness of the cementitious board so that the punches will have passed through the board before the indentors impinge on the board, thus making the through perforations before the indentations. By making the perforations before the indentations, the size of the press required to put the die assembly into operation is kept to a minimum.
It is also preferred that the die assembly includes a stripper plate and a die plate between which a tile is sandwiched to be perforated. The stripper plate has holes therethrough to allow the punches and indentors, if any, to pass through the plate and into the tile, and the die plate has holes therethrough for the parage of the punches after they have perforated the tile.
The punches and indentors may be formed of groups of adjacent cylindrical punch pins or by single punches or indentors of the desired shape. The holes in the punch plate to receive the punch pins or the punches and indentors can be formed by a wire erosion technique. Alternatively, the punches and indentors may be of a ceramic material set into a binder on the surface of the punch plate.
Also according to the invention there is provided a suspended ceiling comprising the tiles of the invention. Such a ceiling can have non-uniform acoustic properties and a substantially uniform appearance by using a mixture of tiles according to the invention and tiles of similar appearance having no through perforations but only fissure-like indentations. Ceilings can thus be made having desired overall acoustic properties; for example a ceiling can be made which is particularly suitable for an auditorium where speech must be clearly audible throughout.